High-precision synchronization of pulsed gas-discharge lasers

ABSTRACT

Two excimer lasers have individual pulsing circuits each including a storage capacitor which is charged and then discharged through a pulse transformer to generate an electrical pulse, which is delivered to the laser to generate a light pulse. The time between generation of the electrical pulse and creation of the light pulse is dependent on the charged voltage of the capacitor. The capacitors are charged while disconnected from each other. The generation of the electrical pulses is synchronized by connecting the capacitors together for a brief period after the capacitors are charged to equalize the charging voltages. The capacitors are disconnected from each other before they are discharged.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to spatial and temporaloverlapping of pulses multiple pulsed gas-discharge lasers. Theinvention relates in particular to spatial and temporal overlapping ofpulses from two or more excimer lasers or molecular-fluorine lasers.

DISCUSSION OF BACKGROUND ART

Excimer lasers are pulsed gas lasers that deliver radiation in theultraviolet (UV) region of the electromagnetic spectrum. There areapplications of such lasers, for example, laser annealing, that wouldbenefit from a pulse-energy greater than available from the highestenergy excimer laser presently available. A greater pulse-energy can besupplied by combining the output of two or more excimer lasers. Theoutput of the lasers must be overlapped spatial and temporally. Thespatial overlap can be achieved precisely using an arrangement ofoptical elements, and optical overlap methods are well known in the art.

The temporal overlap is less precise and depends on the precision whichoperating characteristics of pulsed power supplies, among other factors,can be reproduced from pulse to pulse. Variation of such characteristicsleads to temporal variation of the precision of pulse-overlap. Thistemporal variation is usually referred to by practitioners of the art asjitter. Depending on the pulse duration and on application requirements,there is usually a jitter value that can not be exceeded withoutcompromising the application.

By way of example, in a laser annealing application, it may be requiredto combine the pulsed output of two excimer lasers having a pulserepetition rate of about 600 Hertz (Hz) and a pulse-duration of about 30nanosecond (ns) full-width at half-maximum (FWHM), with pulses having acomplex temporal pulse-shape. Maintenance of a complex temporalpulse-shape is possible only if the pulses are overlapped precisely intime. The tolerance of an application toward variations in the temporalpulse-shape in the overlapped beam cannot be stated in general, anddepends on the sensitivity of the application with respect to the pulseshape. For applications in the laser annealing area, this tolerancelevel corresponds to jitter of a few nanoseconds, for example less thanabout 6 ns for the 30 ns pulse-duration.

In order to understand the factors affecting jitter, it is useful toconsider characteristics of pulsing circuit arrangements for an excimerlaser. A description of one such circuit arrangement is set forth belowwith reference to FIG. 1, which is a circuit diagram depicting onetypical arrangement 10 of circuitry for delivering high voltage pulsesto discharge electrodes of an excimer laser.

Here, high-voltage power (HV-IN) power from a high-voltage power supply(HVPS) is supplied to a terminal 12. The power is used to fully charge astorage capacitor C0 to a predetermined voltage, typically between about1500 and 2300 Volts (V). The capacitor is charged via a magneticisolator 14. Magnetic isolator 14 includes a diode D2 and a transformerL6, one side which is connected to a switch 16 including anisolated-gate bi-polar transistor (IGBT) and rectifier bridge, and othercomponents (not shown). An inhibit signal can open or close switch 16 asrequired.

Magnetic isolator 16 switches the impedance value between the HVPS andstorage capacitor by a factor of about 50 from a low value to a highvalue (and vice versa) depending on whether switch 16 is respectivelyclosed or open. The low impedance value is needed to charge capacitor C0with high precision. The higher impedance value is necessary to protectthe power supply from energy reflected back from the laser discharge,which could otherwise cause very high and potentially destructive peakcurrents through the power supply. Only sufficient description ofmagnetic isolator 14 is provided here to understand the operation ofcircuitry 10. A detailed description of the magnetic isolation principleis provided in U.S. Pat. No. 6,020,723, assigned to the assignee of thepresent invention, and the complete disclosure of which is herebyincorporated herein by reference.

When charging of capacitor C0 is complete, magnetic isolator 14 isswitched to the high impedance condition. Discharging of capacitor C0 iscontrolled by an IGBT and diode (“free-wheeling diode”—FWD) moduleIGBT-1. On receipt of a pulse-trigger voltage at the gate of IGBT-1 theIGBT is closed and capacitor C0 is discharged through a magnetic-assistL5, a pulse-transformer L4, and a diode D1. The resultant pulse fromtransformer L4 is sent to a pulse-compressor 18. The pulse is compressedin three stages formed by saturable inductor or magnetic switch L1 andcapacitor C1, saturable inductor L2 and capacitor C2, and saturableinductor L3 and capacitor C3. The compressed pulse is delivered frompulse compressor 18 to the excimer laser tube which includes the laserdischarge electrodes and other electrically reactive components.

A reset signal is applied to terminal 20 from a DC power supply (notshown). The signal causes a current of about 10 amps (A) to flow throughL5, L1, L2, and L3. This current effectively drives the magnetic coresof these devices from a position in the B-H (hysteresis) loop thereoffollowing a pulse compression, back into one corner of the B-H loop. Asufficient reset is a precondition for obtaining reproducible transitiontimes between one pulse and the next through the pulse-compressor andminimizing jitter between pulses.

Considering now problems that would be encountered in trying to drivetwo excimer lasers, each with a circuit arrangement similar to that ofarrangement 10 of FIG. 1, one prerequisite for a low jitter time betweenpulses delivered by the pulse compressor 18 of each is that the chargedvoltage of storage capacitor C0 of each must be as reproducible aspossible from pulse to pulse. These storage capacitors are each chargedin approximately 1 millisecond (ms) via the HSVP, which may be regardedas a regulated current source. The achievable control precision (voltageregulation accuracy) of the high voltage of typical such HSVPs is about±0.1% of the maximum high-voltage value of the power supply, which isusually about unit of 2.3 kilovolts (kV). A typical controlled value isabout 1.6 kV, i.e., well below the maximum.

The voltage-time area (the magnetic saturation flux) of transformercores of stages of pulse compressors 18 is essentially a constant, withonly a small drift or variation resulting from a change in temperatureof the core material. These variations are sufficiently slow to enablerelatively easy correction for example by controlling the relationshipof independent discharge-trigger signals of the circuits. The saturationflux (Ψ) of a compressor stage can be represented by the followingequation:

Ψ=N∫2B_(s)dA=∫Udt=const.  (1)

wherein N represents the number of turns of the respective compressorstage saturable inductor; B_(s) is the saturation-induction of the corematerial used; and A is the magnetically-effective core cross-section.

The required time for saturation of the core, and hence transition to alow-inductive state, is obtained from the integral of Udt (thevoltage-time area), wherein U is the voltage over this saturableinductivity, which, in turn, is proportional to the charging voltage atthe capacitor C0. It follows from this that a variation of this voltageleads directly to a change in the saturation time (the time for thethrough-connection of the inductances L5, L1, L2, and L3), with thechange in time being about 1/U. In the example under consideration, avoltage change of as small as 1 V at capacitor C0 would lead to a timechange of between about 5 and 7 ns for the pulse passage through theentire pulse compressor 18.

Because each of the two lasers to be synchronized has an independenthigh-voltage power supply unit, relative voltage fluctuations in a rangeof 4.6 V can occur. This would lead to up to 32-ns fluctuations in thetime difference between the light pulses delivered by each of thelasers. This is a random pulse-to-pulse phenomenon and can not bepredicted, and accordingly can not be corrected.

In theory at least, the above described jitter problem could berectified by using HVPSs which would allow a pulse to pulse voltagefluctuation (at each C0) of less than about 0.015%. Realization of suchpower supplies, however, is technologically achievable only with greatdifficulty and at significant expense. This is because of required highcharging power of approximately 50 kilowatts (kW) and charging voltageson the order of 2 kV. There is a need for a solution to the excimerlaser synchronization problem that does not require the development ofimproved power supplies or any other laser components.

SUMMARY OF THE INVENTION

The present invention is directed to electrical apparatus for energizingfirst and second gas-discharge lasers. In one aspect the apparatusincludes first and second storage capacitors and first and secondhigh-voltage power sources arranged to charge respectively the first andsecond storage capacitors. First and second pulse-forming circuits arearranged to discharge respectively the first and second storagecapacitors. The discharging of the capacitors generates respectivelyfirst and second electrical pulses which are delivered to respectivelythe first and second gas discharge lasers. A switching arrangement isprovided and arranged to connect together the first and second storagecapacitors after the capacitors are charged for a predetermined timeperiod prior to the discharge of the capacitors by the pulse-formingcircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain principles of the presentinvention.

FIG. 1 schematically illustrates a prior-art arrangement for energizinga pulsed gas discharge laser including a storage capacitor charged by ahigh voltage power supply, and pulse forming circuitry arranged todischarge the capacitor through a pulse transformer to form anelectrical pulse, electrically compress the pulse, and deliver thecompressed pulse to the gas discharge laser.

FIG. 2 schematically illustrates apparatus in accordance with thepresent invention including first and second energizing arrangementseach similar to the arrangement of FIG. 1, and an arrangement fortemporarily connecting the storage capacitors of each after thecapacitors are charged but before the capacitors are discharged.

FIG. 3 is a timing diagram schematically illustrating one example of theoperation of the apparatus of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Continuing now with reference to the drawings, wherein like features aredesignated by like reference numerals, FIG. 2 is an electrical circuitdiagram schematically illustrating a preferred embodiment 10 ofapparatus in accordance with the present invention for synchronousoperating two excimer lasers (not graphically depicted). The lasers arereferred to in FIG. 2 as laser A and laser B. Lasers A and B areenergized, individually, by laser pulsing arrangements 10A and 10Brespectively, each similar to the above-described prior-art pulsingarrangement 10 of FIG. 1.

Each pulsing arrangement includes a high-voltage power supply 32charging a storage capacitor C0 (CO_(A) and CO_(B)) via a magneticisolator 14. An electrical pulse is generated by commanding IBGT-1, viaa trigger-voltage applied to the gate thereof, to discharge capacitor C0through a pulse transformer L5 (as discussed above). The magnetic pulsecompressor 18 temporally compresses the pulse, and delivers thecompressed pulse to the laser discharge-electrodes. Common features ofthe arrangements 10A and 10B are identified by suffixes A and B appliedto the corresponding reference numeral. Apparatus 30 is controlledsoftware in a PC or the like (not shown) which provides all controlsignals referred to hereinbelow.

As discussed above, absent any pulse-to-pulse charging-voltagedifferences between storage capacitors CO_(A) and CO_(B), optimalsynchronization of the output of the lasers, and correspondingminimization of jitter, could be accomplished by taking into account thedifferent transit-times for a pulse generated by discharge of acapacitor, and by synchronizing pulse-trigger signals Trigger-A andTrigger-B.

In apparatus 30, fluctuations of pulse-to-pulse charging-voltagedifferences are minimized, as summarized above, by connecting capacitorsCO_(A) and CO_(B), together, after the capacitors are charged, for ashort period before a pulse is triggered by either Trigger-A orTrigger-B. The connection is established by a switching arrangement 40including IGBT modules 42A and 42B, which are driven by drivers 44A and44B respectively. An IGBT protection and control circuit 46 generatesdigital signals for the drivers, responsive to a digital connect signalfrom the software controlling the apparatus. Circuitry 46 also monitorsthe voltages of capacitors and is arranged to prevent turning on IGBTs42A and 42B if the voltage difference between the capacitors exceeds apredetermined level, for example 100 V. This serves to protect the IGBTsin the event that one of HSVPs malfunctions, and the correspondingcapacitor is not charged or not sufficiently charged. Circuitry 40 canbe referred to as an “equilibrium switch” or EQUI-Switch.

IGBTs 42A and 42B are connected as depicted in FIG. 2 in an anti-serialmanner. This provides that the IGBTs are able to switch both positiveand negative polarities of the capacitors. This also provides thatlasers A and B can be operated separately from each other, and beindependent of the relative adjustment of power supplies HVPS-A andHVPS-B.

The power supplies are regulated such that one of the capacitors, forexample capacitor CO_(A), is initially charged to a higher voltage thanthe other (CO_(B)). In general terms, the voltage difference must behigh enough, so that under worst condition the difference is stillhigher than sum of the flux voltage of one IGBT switch plus the fluxvoltage of the internal freewheeling diode of the other (anti-serial)IGBT and the residual voltage difference at which the HVPS with thelower voltage takes advantage of the fluctuation upward and the powersupply unit with the higher voltage takes advantage of the fluctuationdownward, i.e., the voltage-regulation accuracy of the power supplies.As noted above, this is about 4.6 V (±2.3 V) in the example underconsideration. Only under this condition will there always be chargeequalization, with the equalization current always flowing in the samedirection. The voltage difference, however, should not be too high,otherwise the equalization may take longer than is practical (>100 μs inthis example) or the equalization current may be too high. A voltageabout 15 V higher has been determined to be adequate, in the exampleunder consideration.

The impedance (R) of the connection between CO_(A) and CO_(B) is chosen(if necessary, by putting additional resistance in series with IGBTs 42Aand 42B) to satisfy a condition

R≧2*(L/C)^(0.5)  (2)

where L is mainly cable inductance and

C=C0_(A) *C0_(B)/(C0_(A) +C0_(B))  (3)

The resulting equalizing current of several tens of amps ensures thatvoltages of capacitors CO_(A) and CO_(B) adjust aperiodically, withinapproximately 50 microseconds (μs), up to the flux voltages of the IGBToperated in the forward current direction and of the internalfreewheeling diode of the other respective IGBT.

Although, as noted above, a residual voltage determined by theregulation accuracy of the HVPSs always remains as the difference in thetwo charging capacitor voltages, it is reproducible and does notcontribute to the temporal jitter. Because of this, the charge voltagesof CO_(A) and CO_(B) still fluctuate absolutely by about ±2.3 V, butrelative to each other by less than 200 mV. This means that, althoughthe gas discharges and, as a result, the light pulses of lasers A and Bjitter against the discharge trigger signal of each, the lasers achievea temporal stability in the nanosecond range relative to each other.Realizable values lie between about 2 ns and 5 ns peak-peak.Accordingly, the temporal shape of the two spatially and temporallyoverlapped light pulses, largely corresponds to the temporal shape ofthe individual pulses. This provides that interaction of the overlappedpulses with a material being processed thereby takes place in the samemanner as for any one of the individual pulses having twice the energy.

A description of the relative timing of signals operating apparatus 30as discussed above is next set forth with reference to the timingdiagram of FIG. 3 and with continuing reference to FIG. 2. As notedabove, these signals are generated or triggered by control software forapparatus 30. In FIG. 3, with an exception of the connection signal, allsignals or values are specific to one of the lasers, here, arbitrarilyselected as laser A. Signals and values for the other laser willtemporally evolve in the same manner. The evolution time of the diagramof FIG. 3 is slightly greater that one pulse-repetition period, here,assumed to be greater than about 1.67 milliseconds (ms) representativeof a pulse repetition frequency of 600 Hz or less.

The control software generates a period trigger signal at time t₀, bywhich all others are timed. At time t₀, the inhibit signal (Inhibit A)applied to magnetic isolator 14A goes from low to high, putting theisolator in a low impedance state to facilitate charging. At this timealso, a signal HVA commands power supply HVPS-A to charge capacitorCO_(A) to the predetermined voltage for that capacitor. The capacitor isnominally charged at a time t₁, but charging continues to a time t₂ totake into account possible pulse-to-pulse differences in charging time.Values for the period t₀ to t₁ and t₁ to t₂ consistent with the exampleunder consideration are less than or equal to about 1040 microseconds(μs) and 100 μs respectively. At time t₂ the inhibit signal goes fromhigh to low, putting the magnetic isolator in a high impedance state toeffectively isolate capacitor CO_(A) from power supply HVPS-A.

A relatively short time after time t₂, for example, about 10 μs after,the connect signal goes from low to high, closing IGBTs 42A and 42B (theEQUI-Switch) so that the above-described voltage equalization betweencapacitors CO_(A) and CO_(B) can take place. A relatively short timebefore time t₃ (here again about 10 μs), the connect signal goes fromhigh to low opening EQUI-Switch and isolating the capacitors from eachother so that capacitors can be independently discharged. At time t₃,the trigger signal closes IGBT-1A for a period long enough to dischargecapacitor CO_(A), an electrical pulse (not shown) is generated andcompressed, and corresponding light output pulse is delivered from thelaser a few microseconds later. During a recovery period between timest3 and t4, after IGBT-1A is re-opened the voltage of capacitor CO_(A)jumps up slightly, due to charging by reflected energy from thedischarge due to less than perfect impedance matching, then driftsgradually down to about the original uncharged value by time t₅ at whichtime a new sequence of signals is triggered. Here, it should be notedthat for a PRF of 600 Hz the time period between t4 and t5 would berelatively short. However, recharging could actually start at time t4.

A reason for closing the EQUI-Switch shortly after time t₃ and openingthe EQUI-switch shortly before time t₃ is that the EQUI-Switch is commonto both lasers. The difference in the EQUI-Switch closed time (about 100μs) and the period t₃−t₂ allows for the relative trigger times of thelasers to be varied to compensate for any above-discussed relative driftin pulse-propagation time through pulse-compression circuits 18A and18B, thereby optimizing temporal overlap of the corresponding lightpulses.

It is emphasized, here, that the embodiment of the present inventiondescribed above and circuitry and values used are merely one example andshould not be construed as limiting the present invention. Those skilledin the electrical arts, from the description of the present inventionprovided above, may devise other circuitry for providing the inventivevoltage-equalization function without departing from the spirit andscope of the present invention. Further, while the present invention hasbeen described in terms of synchronizing the output of two lasers withindependent pulsing arrangements, the invention could be extended tosynchronizing three or more lasers with independent pulsingarrangements. By way of, example, three lasers with independent pulsingarrangements could be synchronized using two of the EQUI-switcharrangements described herein.

In summary, the present invention is described above in terms of apreferred embodiment. The invention however is not limited to theembodiment described herein. Rather the invention is limited only by theclaims appended hereto.

1. Electrical apparatus for energizing first and second gas-dischargelasers, comprising: first and second storage capacitors; first andsecond high-voltage power sources arranged to charge respectively thefirst and second storage capacitors; first and second pulse-formingcircuits arranged to discharge the first and second storage capacitorsto generate first and second electrical pulses and deliver the first andsecond electrical pulses to respectively the first and second gasdischarge lasers; and a switching arrangement arranged to connecttogether the first and second storage capacitors for a predeterminedtime period prior to the discharge thereof by the pulse-forming circuitsfor equalizing charging voltages of the capacitors before the dischargethereof.
 2. The apparatus of claim 1, wherein the time period isselected such that the capacitors are disconnected before thedischarging of the storage capacitors.
 3. The apparatus of claim 1,wherein the pulse-forming circuits are arranged to discharge the firstand second storage capacitors responsive to first and seconddischarge-trigger signals.
 4. The apparatus of claim 1, wherein thestorage capacitors are not connected together during the chargingthereof and the power supplies are arranged to charge the capacitors torespectively first and second voltages before the connecting togetherthereof, with the connecting together period beginning after thecapacitors have been charged to the first and second voltages and endingbefore the discharge of the capacitors.
 5. The apparatus of claim 4,wherein the first voltage is greater than the second voltage.
 6. Theapparatus of claim 5, wherein the power supplies are regulated powersupplies having a voltage-regulation accuracy and wherein the differencebetween the first and second voltages is selected cooperative with theswitching system and the voltage regulation accuracy of the powersupplies such that during the connecting together period of thecapacitors current can only flow from the first capacitor to the secondcapacitor.
 7. The apparatus of claim 6, wherein first and secondvoltages are between about 1500 Volts and 2300 Volts, thevoltage-regulation accuracy of the power supplies is about plus or minus2.3 Volts, and the difference between the first and second chargingvoltages is about 15 Volts.
 8. The apparatus of claim 1, wherein theswitching arrangement includes first and second IGBT-diode modulesconnected in an anti-serial manner with each other and with the firstand second IGBT-diode modules connected respectively with the first andsecond storage capacitors.
 9. The apparatus of claim 8, wherein thefirst and second storage capacitors are connected and disconnected byrespectively, closing and opening the IGBT-diode modules.
 10. Theapparatus of claim 1, wherein the first and second pulse formingcircuits include first and second pulse transformers through which thefirst and second storage capacitors are discharged to generate therespectively first and second electrical pulses and first and secondpulse compressors for temporally compressing the first and secondelectrical pulses before the first and second electrical pulses aredelivered to the respectively first and second gas-discharge lasers. 11.Electrical apparatus for energizing first and second gas-dischargelasers, comprising: first and second storage capacitors; first andsecond high-voltage power sources arranged to charge respectively thefirst and second storage capacitors to respectively first and secondvoltages; first and second pulse-forming circuits arranged to dischargethe first and second storage capacitors to generate first and secondelectrical pulses and deliver the first and second electrical pulses torespectively the first and second gas discharge lasers; and a switchingarrangement arranged cooperative with the first and second storagecapacitors such that the first and second storage capacitors aredisconnected from each while being charged, connected together for apredetermined time period after being charged for equalizing chargingvoltages thereof, then disconnected from each other prior to thedischarge thereof by the pulse-forming circuits.
 12. The apparatus ofclaim 11, wherein the switching arrangement includes first and secondIGBT-diode modules connected in an anti-serial manner with each otherand with the first and second IGBT-diode modules connected respectivelywith the first and second storage capacitors, and wherein the first andsecond storage capacitors are connected and disconnected byrespectively, closing and opening the IGBT-diode modules.
 13. Theapparatus of claim 12, wherein the first voltage is greater than thesecond voltage.
 14. The apparatus of claim 13, wherein the powersupplies are regulated power supplies having a voltage-regulationaccuracy and the IGBT-diode modules have a flux voltage and wherein thedifference between the first and second voltages is selected to exceedthe sum of the flux voltage and the voltage regulation accuracy of thepower supplies such that, during the connecting together period of thecapacitors, current can only flow from the first capacitor to the secondcapacitor.
 15. The apparatus of claim 14, wherein first and secondvoltages are between about 1500 Volts and 2300 Volts, thevoltage-regulation accuracy of the power supplies is about plus or minus2.3 Volts, and the difference between the first and second chargingvoltages is about 15 Volts.
 16. A method of operating first and secondpulsed gas discharge lasers, the first and second lasers including,respectively, first and second storage capacitors, first and secondhigh-voltage power sources for charging respectively the first andsecond storage capacitors, and first and second pulse-forming circuitsincluding respectively first and second pulse transformers through whichrespectively the first and second capacitors can be discharged togenerate respectively first and second electrical pulses and deliver thefirst and second electrical pulses to respectively the first and secondgas discharge lasers, the method comprising the steps of: charging thefirst and second capacitors to respectively first and second voltages,with the first and second capacitors disconnected from each other;following the charging step, connecting the first and second capacitorstogether for a predetermined time period; at the end of thepredetermined time period, disconnecting the first and second capacitorsfrom each other; then discharging the first and second capacitorsthrough the first and second pulse transformers to provide the first andsecond electrical pulses.
 17. The method of claim 16, wherein the firstvoltage is higher than the second voltage.
 18. The method claim 17,wherein the first and second power sources are regulated power supplieshaving a voltage regulation accuracy, the first and second capacitorsare connected together by semiconductor switch arrangement having a fluxvoltage when current passes therethrough, and wherein the differencebetween the first and second voltage is selected to be greater than thesum of the voltage accuracy and the flux voltage, whereby during theconnecting step current flows only from the first storage capacitor tothe second storage capacitor.
 19. The method of claim 18, wherein firstand second voltages are between about 1500 Volts and 2300 Volts, thevoltage-regulation accuracy of the power supplies is about plus or minus2.3 Volts, and the difference between the first and second chargingvoltages is about 15 Volts.